Three dimensional printers can create physical models from digital designs. While current printers can use a variety of different colors of build materials for greater flexibility in realizing aesthetic or functional objects, attempts to mix different colors have generally provided unsatisfactory results in extrusion-based processes due in large part to the highly viscous nature of thermoplastics and other typical build materials. There remains a need for color mixing in three dimensional printing to provide greater user control over the look of fabricated items.